How long can the car sit before a relatively new (year old) battery goes dead? And if you are going to start the car periodically, how long does it need to run to give the alternator a chance to recharge the battery? TIA...

 

It really depends on what draws youhave in the car. I would assume 6 months but I have no basis to back that up. Car batteries in Florida only last 2 or so years anyway.

If you were to start the car and run it maybe once a month, maybe take it down the road to a store or something once a month I would guess you would be ok.

 

Are you going out of town and need someone to tend to your car while you are gone? I am right down the street from you... maybe 10 miles away.

 

If the battery is in good condition and there is no abnormal current drain, a fully charged battery should be able to start the car after sitting two months or longer. If the engine tune allows the car to start quickly (within 5-10 revolutions of the crank), you could get almost 4-5 months before it may not start.

In any event, the alternator is not really up to the task of charging a battery back from a condition where it would not start the car. The alternator internals would overheat and probably fail after a couple time of recharging a battery. The best thing to have is a battery maintainer/trickle charger that keeps the battery at a full-charge state.

 

Much more effective to buy a Battery Tender Jr. (about 20 dollars and leave it on all the time....It will start first time every time and keep the battery in top shape......

 

You are a generous and thoughtful fellow indeed!

Thanks for the replies but it's actually for a snowbird who asked me to periodically start his car so that when he returns to town in November, the battery isn't dead. I was wondering how often I had to start it and for how long I'd have to let it run so that it would start for him once he's back down here. Unfortunately, the car's stored outside and so there's no way to run a trickle charge. I was thinking of just pulling the battery and reinstalling it in October but he's the kind of guy who'd notice all his preprogrammed radio stations are no longer and I don't feel like reprogramming them for him or putting any more energy into this than I have to....

 

Up here in Michigan during the Winter Months, I start and let the Vette run for an Hour... No problem with Battery running down..

 

You know, I bought my car and had no idea how old the battery in it was. In the first year and a half, I drove it a total of about 1500 miles. Over the winter months it sat for almost four months straight without starting it. All that time, whenever I went out to start it, it never had a problem starting. Finally, after about a year and a hlaf, I noticed it cranking slower and the next season, it was dead.

I put a brand new battery in, drove it through the season, put it down for the winter in mid-October. I went to fire it up the first week in December so I could get at the artifical tree it was blocking in the garage and much to my surprise, it was dead! My old battery (unknown age) had lasted a year and a hlaf through the torture I was giving it, but the brand new one (a Napa battery with a 72 month warranty) couldn't hack a month and a half!!!

I bought a trickle charger and have been using it ever since. One of the best purchases I've ever made.

You say the car is sitting outside, but how far is it from an electrical outlet?? Can you reach it with a good 3-wire (grounded) extension cord? I bought an actual "Battery Tender" brand charger. Yes, it was more expensive then a lot of others out there, but it was for my Vette, I didn't want to skimp It came with two sets of leads...

One set has alligator clips where you obviously leave the clamshell up, and connect to the positive terminal and a ground and it does it's thing...

The other lead basically gives you the option of making a more 'permanent' connection by bolting them right to the terminals. You then have a small cord which dangles down the inner fender where you can just get to it by reaching under the car. When you want to connect it to the trickle charger, you simply reach under the car and plug the charger into that cable lead. This makes it simple to keep the battery charged even if the car is stored under a cover and you don't want to have to remive it...

Just my $ .02

 

Lead acid batteries self discharge 1% each day disconnected and if you draw normal leakage current, they will discharge at a higher rate. GM says leakage current should not exceed 50 ma. Since there is a large variation in battery performance and amp-hour capacity and car to car leakage current, it would be hard to tell how long the battery can sit before it won't start a car. Very roughly, cranking for 4 seconds uses about 1/10 amp hour of capacity and the alternator , also very roughly, should be able to return 1/10 amp hour in 2 to 3 minutes, but don't count on that figure for your particular car!!!
Here's how you can determine how long it takes to recharge the cranking energy. First, after the car has sat for at least 24 hours unused, measure the battery terminal voltage. Lead acid batteries are considered discharged at 12.0 volts or lower and fully charged at 12.9 volts or higher and linear in between, example, 12.45 volts, 50% charged. After starting the engine and idling for the period you think is appropriate, let the car sit unused for 24 hours and then measure the battery terminal voltage again (using a test voltmeter, not the dash voltmeter) and compare that reading with the one you recorded before you first started the engine. A lower 2nd reading means the battery wasn't recharged, higher means the cranking amp-hours were more than replaced.
When lead acid batteries sit unused and self discharge, they collect lead sulphate on their plates and lead sulphate is a good insulator. A fully sulphated battery is a door stop! A better way to keep the battery up and not generate lead sulphate is to use a battery tender, or if you have a battery charger, buy an interval timer (turns your home lights on each night when you aren't home) and have it charge the battery each day for its minimum interval (usually 15 mins). The battery will have a normal life span. Remember, when you start the engine and don't let it run long enough to fully warm up and also heat the exhaust up, moisture in the engine oil and moisture collected in the exhaust don't get evaporated and this causes sludge in the engine oil and rust in the exhaust system. I recommend you charge the battery instead of running the engine for the minimum time to recover the cranking amp-hours.